Water use and production potential of Karoo shrubs

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Date
2015-06
Authors
Malan, Paul Johannes
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Publisher
University of the Free State
Abstract
English: Variation in, and changing of climatological patterns, especially rainfall, as well as grazing intensity and frequency has the biggest influence on rangeland productivity and sustainable animal production in the Nama-karoo Biome. This study was conducted to quantify whole-plant productivity, nutritive value and morphological adaptations of Karoo shrubs to defoliation along a soil-water deficit gradient. Two Karoo shrubs, Nenax microphylla and Pentzia incana were investigated. The watering treatments included the following: 0 - 25% depletion (non-stressed), 25 – 50% depletion (mildly stressed), 50 – 75% depletion (moderately stressed) and 75 – 100% depletion (severely stressed) of field capacity. The defoliation treatments were defoliation intensity to a height of 50 mm, 125 mm and 200 mm; and defoliation frequencies of three-monthly, six-monthly and twelve-monthly which was also used as the control. Water availability proved to be the single most important factor influencing both above- and belowground rangeland productivity. Defoliation intensity had the lowest impact on productivity, while the impact of defoliation frequency was markedly higher on both above- and belowground phytomass production. The root:shoot ratio increased with increased water deficit as a means to improve the water absorption of the shrubs. Determination of water-use efficiency (WUE) included both above- and belowground phytomass, while it excluded evaporation which gave a more accurate estimation of WUE. This is of the few studies where root growth is also included in calculating WUE. The expression of WUE in terms of transpiration as was done in this study, is more sensitive for describing ecosystem functioning than evapotranspiration, as was done in most studies in the past. In general, the WUE of the shrubs increased when exposed to water stress and higher grazing pressure. The more frequent and intensely the plants were defoliated, the higher the nutritive value of the produced edible phytomass. The quantity of this produced phytomass was, however, very low. The increased CP (N-content) of the water stressed plants could have contributed to the increased WUE. Strong evidence of compensatory growth and WUE were recorded for both species. This compensatory ability especially enabled the shrubs to display increased recovery after defoliation when water is not limited. Water stress had no marked influence on the reproductive ability of the investigated shrubs. It was, however, proved that both defoliation frequency and intensity had a bigger influence on seed production and germination percentage, than water stress. Pentzia incana has a high density of reflective trichomes that provides protection against heat and solar radiation. It also has a high stomatal density which allows increased photosynthetic rates when growth conditions are favourable. The stomata of N. microphylla occur only on the abaxial (lower) side of the leaf which protects it from direct sunlight and heat. It also has a very high stomatal density which could contribute to its ability of compensatory growth when adequate water is available. Furthermore, the leaves have a shiny appearance which enables them to reflect solar radiation to reduce leaf temperatures. This study highlighted the complexity of the effect of external influences, like rainfall and grazing (defoliation) on the functioning of the rangeland ecosystem in the arid and semi-arid Nama-karoo Biome. Although the land user does not have control over plant water availability, control over defoliation is possible. Defoliation therefore should be the most important key aspect in sustainable ecosystem utilization.
Afrikaans: Variasie, en verandering in klimaatspatrone, veral reënval, sowel as intensiteit en frekwensie van beweiding het die grootste invloed op weiveldproduktiwiteit en volhoubare diereproduksie in die Nama-karoo Bioom. Hierdie studie is uitgevoer om die geheel plant se produktiwiteit, voedingswaarde en morfologiese aanpassings van Karoo bossies tydens ontblaring en blootstelling aan ‘n grondwaterstremming gradiënt te kwantifiseer. Twee Karoo bossies, naamlik Nenax microphylla en Pentzia incana is ondersoek. Die waterbehandelings het die volgende ingesluit: 0 - 25% versadiging (nie gestrem), 25 - 50% versadiging (effens gestrem), 50 - 75% versadiging (redelik gestrem) en 75 - 100% versadiging (volkome gestrem) van veldkapasiteit. Die ontblaringsbehandelings het ingesluit, intensiteite van 50 mm, 125 mm en 200 mm hoogtes; en ontblaring frekwensies van drie-maandeliks, ses-maandeliks en twaalf-maandeliks wat ook as die kontrole beskou is. Beskikbaarheid van water is die enkele faktor wat die grootste impak op beide bo- en ondergrondse weiveldproduktiwiteit gehad het. Ontblaring intensiteit het produktiwiteit die minste beïnvloed, terwyl die impak van ontblaring frekwensie merkbaar hoër is op beide bo- en ondergrondse fitomassaproduksie. Die wortel:stingel verhouding het toegeneem met toename in waterstremming om sodoende die water opname van die bossies te verbeter. Vir die berekening van waterverbruiksdoeltreffendheid (WVD) is beide die bo- en ondergrondse fitomassa gebruik, terwyl evaporasie ook uitgeskakel is, wat sodoende ‘n meer akkurate WVD bepaling tot gevolg het. Hierdie is een van die min studies waar wortelproduksie tydens die bepaling van WVD in berekening gebring is. Die uitdrukking van WVD in terme van transpirasie, in plaas van evapotranspirasie soos in meeste studies in die verlede uitgevoer, lewer ‘n akkurater bydrae tot ekosisteem funksionering. Oor die algemeen is daar ‘n toename in WVD indien plante aan waterstremming en beweidingsdruk blootgestel is.Die vreetbare plantmateriaal se voedingswaarde het toegeneem met toename in intensiteit en frekwensie van ontblaring. Die hoeveelheid geproduseerde vreetbare materiaal is egter baie laag. Die toename in ru-proteïn (N-inhoud) van die watergestremde plante het moontlik bygedra tot verbeterde WVD. Sterk tekens van kompensatoriese groei en WVD is waargeneem vir beide plant spesies. Hierdie kompensatoriese vermoë stel die plante in staat tot versnelde hergroei na ontblaring, mits voldoende water beskikbaar is. Waterstremming toon geen merkbare invloed op die reproduksievermoë van die plante nie. Beide frekwensie en intensiteit van ontblaring toon ‘n groter invloed op saadproduksie en persentasie ontkieming as die geval met waterstremming. Pentzia incana beskik oor ‘n hoë digtheid weerkaatsende trigome wat die plant teen hitte en bestraling beskerm. Die plant het ook ‘n hoë huidmondjie digtheid wat versnelde fotosintese onder gunstige klimaatstoestande toelaat. By Nenax microphylla kom huidmondjie slegs op die abaksiale (onder) kant van die blare voor, wat dit teen hitte en direkte sonlig beskerm. Hierdie plant het ook ‘n baie hoë huidmondjie digtheid wat waarskynlik bydrae tot die kompensatoriese groeivermoë wanneer voldoende water beskikbaar is. Die blare van N. microphylla het ‘n blink voorkoms wat dit in staat stel om sonstrale te weerkaats en sodoende blaartemperatuur te verlaag. Hierdie studie het die kompleksiteit van die effek van eksterne faktore, soos reënval en beweiding op die funksionering van die weidingekosisteem in die ariede en semi-ariede Nama-karoo Bioom, beklemtoon. Alhoewel die boer nie beheer oor water beskikbaarheid vir plante het nie, kan ontblaring (beweiding) wel beheer word. Ontblaring is dus die mees belangrike sleutel aspek in die volhoubare benutting van die weidingekosisteem.
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Keywords
Above- and belowground phytomass, Compensatory growth, Defoliation intensity and frequency, Leaf morphology, Nutritive value, Water-use efficiency, Thesis (Ph.D. (Animal, Wildlife and Grassland Sciences))--University of the Free State, 2015, Climate changes -- South Africa -- Karoo, Climatology, Soils and climate -- South Africa -- Karoo, Defoliation
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